Pulses are conventionally characterized by waveform, duration and frequency. The waveform that the pulse must have depends on the intended applications. Some cases may require a perfectly rectangular pulse. In other cases, for example, the waveform for a trigger circuit of an oscilloscope is not important, but a pulse with short duration and a steep rising edge is necessary. It may also be necessary to generate the pulses one by one, or generate the pulses continuously at a preset frequency. Often the duration of the pulse and the repetition rate must be adjustable.
Pulse generation can occur by using differentiating circuits, multivibrator circuits, locked oscillating circuits and delay lines. The most common pulse generation circuit involves the use of preset time constants, i.e., RC circuit, RC cascades or logic gate cascades. Circuits requiring good precision use a clock, if available, or use the time required by a capacitor to charge under a constant current until a preset threshold voltage is reached. In this case, the duration of the pulse is determined by the conventional relation of T=C*V/I.
By choosing a fixed current and a fixed threshold voltage, pulse precision is affected only by the variations in the capacitance related to the process. A fixed current is provided, for example, by extracting a current from a band-gap voltage divided by an external resistance. A fixed threshold voltage is provided, for example, by the K* band-gap voltage. If it is possible to adjust the current to follow the capacitance variation, precision is very high by using, for example, conventional structures such as the one shown in FIG. 1. In this case, the main drawback is the difficulty in generating pulses having a relatively long duration, i.e., on the order of 20-100 microseconds within a chip. The threshold voltage V is limited to the value of the supply voltage, and it is therefore necessary to use an extremely high capacitance value C with very low values of the current I. Accordingly, precision is significantly degraded.